Gas discharge lamps such as metal halide or fluorescent lamps are typically powered from a source of a.c. power. The a.c. voltage, however, is typically first conditioned by a so-called ballast circuit that provides a suitable driving voltage for the lamp to achieve at least a minimum level of lamp performance.
A typical prior art ballast circuit is known as a "lead" ballast circuit, since it provides current to a lamp at a phase angle in advance of, or leading, the voltage supplied to the lamp. Such ballast circuits typically use a capacitor and an inductor serially connected to the lamp. The capacitive impedance of the capacitor significantly exceeds the inductive impedance of the inductor. The aggregate impedance of the serially connected capacitor and inductor is thus capacitive, which causes the current supplied to the lamp to be leading.
It is desirable, however, that the current drawn from the a.c. source have a phase angle close to that of the voltage supplied by the source. This is necessary to achieve a good power factor for the ballast circuit, which power factor is maximized when the current and voltage from the a.c. source are exactly in phase. To address this problem, a power factor-correcting inductor can be connected across the a.c. supply of a lead ballast circuit. Such inductor will draw inductive, or lagging, current from the a.c. source. The aggregate current drawn from the source, which includes the leading current from the mentioned capacitor and inductor of the lead ballast circuit becomes less leading, i.e. more in phase with the voltage supplied by the a.c. source. A high power factor is thus obtained.
The foregoing solution of using a power factor-correcting inductor in a lead ballast circuit, however, fails to remedy a poor level of regulation of power provided to the lamp. Typically, a 10 percent fluctuation in the a.c. supply voltage results in about a 10 percent fluctuation in lamp power. Such fluctuation in lamp power frequently manifests in annoying changes in the light-output level of a lamp. It would, therefore, be desirable to provide a lead ballast circuit for a gas discharge lamp that attains regulation of lamp power to a narrower range, such as 5 percent, for instance.